How does a systemic insecticide act on the pest?

A systemic insecticide functions by being absorbed by the plant and moving throughout its tissues. Once ingested by the pest, such as through feeding on the leaves or stems, the insecticide disrupts the pest's physiological processes, leading to its death. This method of action is particularly effective because it targets the pests that consume the plant material, ensuring that even those that are not present on the surface but are feeding on the plant will be affected.

The other options do not accurately describe how systemic insecticides work. Surface effects, repelling pests, or requiring direct soil contact do not encompass the primary mechanism of action for systemic insecticides, which is their internal movement within the plant and subsequent ingestion by pests.